Femoroacetabular impingement (FAI) is a condition in which abnormal bone shape at the hip joint causes the femoral head and the acetabulum (socket) to make excessive contact during hip motion. This repetitive contact damages the labrum and cartilage and is a leading cause of hip pain in young adults and a recognized precursor to early hip osteoarthritis.
FAI is one of the most important conditions to identify and treat early because its consequences compound over time. Repetitive impingement progressively damages the hip labrum, and labral tears are present in the majority of symptomatic FAI patients. Untreated, the cartilage damage that accompanies FAI is a well-established pathway to early hip osteoarthritis — often in patients in their 30s and 40s. Paralabral cysts frequently develop at the site of labral tears and can be the first visible sign of underlying FAI on MRI.
Causes
Developmental bone shape abnormalities.
FAI results from subtle abnormalities in bone shape that develop during skeletal growth — the hip does not form a perfectly spherical ball or a perfectly round socket, and this geometric mismatch causes abnormal contact at the extremes of motion. Cam deformity is characterized by extra bone at the junction of the femoral head and neck, creating an aspherical femoral head that cannot rotate smoothly within the socket. Pincer deformity involves excessive bony coverage of the femoral head by the acetabular rim, causing the rim to impinge against the femoral neck with flexion. Many patients have a combination of both deformities — so-called mixed FAI — which is the most common pattern overall.
Contributing factors.
High-level athletic participation during adolescence — particularly hockey, soccer, football, and basketball — has been associated with the development of cam-type FAI, as repetitive end-range hip loading during skeletal maturation appears to stimulate abnormal bone remodeling at the femoral head-neck junction. This has important implications for youth sports participation guidelines. Family history and certain developmental hip conditions such as Legg-Calvé-Perthes disease and slipped capital femoral epiphysis also increase the risk of developing FAI morphology.
Symptoms
The classic symptom is deep anterior groin pain that worsens with prolonged sitting, squatting, athletic activity, or hip flexion. Patients frequently use the "C sign" — cupping the hand around the lateral hip to indicate a deep, circumferential pain source — which is a useful clinical indicator of intra-articular pathology. Clicking, catching, or locking sensations are common, particularly when a concurrent labral tear is present. Progressive stiffness and reduced range of motion — especially hip flexion and internal rotation — are characteristic findings that distinguish FAI from extra-articular causes of hip pain. Symptoms often begin insidiously and may be present for months to years before the diagnosis is made, particularly in patients who attribute groin discomfort to a muscle strain.
Diagnosis
A physical exam by a doctor includes the anterior impingement test (FADIR — Flexion, ADduction, Internal Rotation), which reliably reproduces groin pain in FAI, and a thorough range of motion assessment in all planes. A diagnostic intra-articular injection of local anesthetic that temporarily abolishes the pain strongly implicates intra-articular pathology as the pain source. X-rays identify the characteristic bone shape abnormalities — the pistol grip deformity of cam morphology and the crossover sign or coxa profunda of pincer morphology — and allow measurement of specific angles including the alpha angle (cam severity) and the lateral center-edge angle (acetabular coverage). An MRI arthrogram is the gold standard for evaluating the labrum and articular cartilage, identifying tears, chondral damage patterns, and paralabral cysts — all essential for surgical planning and for establishing the degree of joint damage that determines whether arthroscopic preservation or arthroplasty is the appropriate intervention.
Impingement classification
Doctors classify FAI by the morphologic pattern of the bony abnormality, as each type produces a characteristic pattern of labral and cartilage injury.
- Cam type: Extra bone at the femoral head-neck junction creates an aspherical femoral head that jams into the acetabular cartilage and labrum with hip flexion. More common in young male athletes. Tends to produce cartilage damage at the anterosuperior acetabulum.
- Pincer type: Over-coverage of the femoral head by the acetabular rim causes rim impingement against the femoral neck. More common in middle-aged women. Tends to produce labral crushing and a contrecoup cartilage lesion on the posterior femoral head.
- Mixed (combined) type: Features of both cam and pincer impingement present simultaneously. The most common pattern in symptomatic patients presenting for surgical evaluation.
Treatments
Treatment depends on the type of impingement, presence and severity of labral or cartilage damage, symptom duration, and patient activity level.
After initial onset of symptoms, rest, activity modification — particularly avoiding deep hip flexion and provocative positions — and anti-inflammatory medications are recommended.
Mild symptoms without significant labral damage: Physical therapy focusing on core strengthening, hip rotator conditioning, and movement pattern correction to reduce dynamic impingement can provide good results in carefully selected patients. Avoiding deep squatting and positions of maximal hip flexion is important during rehabilitation. Intra-articular corticosteroid injections can provide significant pain relief and help patients participate more effectively in physical therapy, though they do not address the underlying bony deformity.
Persistent symptoms in young and active patients
Surgery is often advised for young or active patients with persistent symptoms despite several months of appropriate conservative care, particularly when labral tears or cartilage damage are identified on MRI arthrogram. Arthroscopic hip surgery is the primary approach: femoroplasty reshapes the cam deformity, acetabuloplasty addresses the pincer overcoverage, and the torn labrum is repaired with suture anchors and re-tensioned against the acetabular rim. Chondral defects are addressed with microfracture or biologic augmentation as indicated. After surgery, a structured physical therapy program — typically 4–6 months — is critical to restoring full strength and function.
Advanced arthritis with FAI
When FAI has progressed to advanced hip osteoarthritis with significant joint space loss, arthroscopic surgery is no longer appropriate and total hip arthroplasty becomes the recommended treatment. This underscores the importance of identifying and treating FAI before cartilage damage becomes irreversible — a window that MRI is uniquely positioned to identify before it closes.
Get an MRI to Confirm Your Diagnosis
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